Glass sheet capable of having controlled warping through chemical strengthening

ABSTRACT

The invention relates to a float glass sheet having a boron- and lithium-free glass composition comprising the following in weight percentage, expressed with respect to the total weight of glass: 65≤SiO 2 ≤78% 5≤Na 2 O≤20% 0≤K 2 O&lt;5% 1≤Al 2 O 3 &lt;6% 0≤CaO&lt;4.5% 4≤MgO≤12% a (MgO/(MgO+CaO)) ratio≥0.5 characterized in that the glass sheet has: (I). The invention corresponds to an easy chemically-temperable soda-silica type glass composition, which is more suited for mass production than aluminosilicate glass, and therefore is available at low cost, and with a base glass/matrix composition that is close to or very similar to compositions already used in existing mass production, and finally which shows reduced or controlled increased warping effect.

1. FIELD OF THE INVENTION

The present invention relates to an improved glass sheet which is ableto be chemically tempered/strengthened and capable of having controlledwarping through chemical strengthening. For example, the presentinvention relates to a glass sheet capable of being inhibited fromwarping through chemical strengthening to keep its flatness, oralternatively, capable of being warped through chemical strengthening toa desired shape.

In particular, the present invention relates to an improved glass sheetwhich is able to be easily chemically tempered/strengthened, capable ofhaving controlled warping through chemical strengthening, and which isinexpensive and easy to produce.

Chemically strengthened glass sheets are finding increasing applicationsin specialized glazing jobs where a mechanical resistance isrequired/mandatory, in a monolithic or laminated form, liketransportation (i.e. aeronautical, automotive), building/architectureand display industries. Amongst such applications, the display industryhas become in the several past years a huge market on demand forchemically strengthened transparent glass sheets as protective/coverglass, viewing window or (touch)screen for numerous electronic deviceslike mobile phones, smartphones, TV, computers, digital cameras, etc.Indeed, as many of these devices are portable, the glass used ismechanically solicited a lot and it is therefore highly desirable thatit is able to tolerate impact and/or damage, such as scratches orimpact, during use and transport. Chemical strengthening is even more ofgreat importance in the domain of displays because such a domainrequires glass sheets of low thickness (as low as less than 1 mm) andbecause chemical strengthening is known as the process of choice tomechanically reinforce (ultra-)thin glass sheets. For weight reasons, itis also advantageous to use thin glass sheets as cover glass for solar,thermal or photovoltaic device.

2. SOLUTIONS OF THE PRIOR ART

The chemical strengthening of a glass article is a heat inducedion-exchange, involving replacement of smaller alkali sodium ions in thesurface layer of glass by larger ions, for example alkali potassiumions. Increased surface compression stress occurs in the glass as thelarger ions “wedge” into the small sites formerly occupied by the sodiumions. Such a chemical treatment is generally carried out by immergingthe glass in an ion-exchange molten bath containing one or more moltensalt(s) of the larger ions, with a precise control of temperature andtime. The rupture strength of a glass article which has been so treatedis thus increased by a value approximately equal to the surfacecompressive stress generated.

Nevertheless, a damage capable of affecting the surface of a chemicallystrengthened glass during its use leads to a decrease in thisstrengthening effect and can even annihilate it if the damage is suchthat the layer under compression is penetrated. In consequence,depending on the use intended for the chemically strengthened glass,focus is made on achieving a high value of surface compressive stress(or “CS”) and/or a high value of thickness of the layer undercompression (which is associated with the parameter called the “depth oflayer” or “DoL”, namely the depth reached by the ions introduced) whichis ideally at least equal to the depth of the largest possibledefect/damage that the glass may undergo. The combination of these twoparameters are generally considered to define appropriately the qualityof the resulting mechanical strength.

In particular, in the display domain, when using a “piece-by-pieceprocess” to produce chemically strengthened glass sheets (cutting tofinal size is carried out before tempering treatment), a high value ofDoL (preferably higher than 10 microns and very preferably higher than12 microns or even better higher than 15 microns) is searched for edgestrength, while when using a “sheet process” (cutting to final size iscarried out after tempering treatment), “central tension” (defined as(CS*DoL)/(glass thickness−2*DoL)) must be kept low.

It is also known that the two strengthening parameters also dependsignificantly, for a given glass composition, on the conditions oftemperature and time of the ion exchange process. Thus, the thickness ofthe layer under compression increases with the temperature and with theduration of the ion-exchange according to the known diffusion laws. Butthe higher the temperature, the more rapidly the stresses induced by theion exchange relax. Likewise, extending the treatment for a too longperiod allows giving the stresses the necessary time to relax and thusresults in a less degree of toughening. The conditions to be chosen forthe process therefore reside generally in a compromise between theoptimum temperature and the minimum duration, to optimize process cost.

To lower the cost of the chemical strengthening (limiting durationand/or temperature to reach searched values of compressive stress andDOL), a lot of glass compositions which are “easy chemically temperable”(meaning that they especially favour ion exchange) have been proposed(merely described or already on the market) but they generally havevarious drawbacks.

Many of them comprise ingredients originating from expensive rawmaterials and/or considerably modifying the physical properties of theglass (molten or final). Some of the chemically temperable glasscompositions known contain, for example, significant contents of lithiumand/or boron. However, lithium has the disadvantage of increasing thedensity of the glass while boron has the disadvantage to cause sometimesformation of ream by its evaporation and furnace wall/refractoriescorrosion. Moreover, both have the additional and significant drawbackto greatly increase final glass price, due to high price of theircorresponding raw materials.

Aluminosilicate-type glass compositions, such as for example thosedescribed in US Patent Application US2012/0196110 A1, the GORILLA® glassproduct from Corning or the DragonTrail® glass product from Asahi GlassCo., are also known to be very efficient for chemical tempering.However, they have a lot of drawbacks. Their high temperature propertiesmake them very difficult to produce (viscosity, fining ability, forming,refractories corrosion). Their cost is relatively high due toexpensiveness of some raw materials to use (i.e. alumina) and due to thehigh temperatures required for their production (high content ofenergy/fuel).

Contrary to aluminosilicate glass compositions, soda-lime-silica glasscompositions are generally not considered as good candidates for easychemically temperable compositions, even if they are by far lessexpensive.

Finally, it is known that it is quite difficult to modify, evenslightly, a glass composition, because:

-   -   a glass production line, and in particular a float line,        represents considerable investment and it is not easily        repairable if the composition causes, for example, damages to        the refractories; and    -   the transition time while changing from a composition to another        is one parameter which is of high importance when producing        glass, because if long, the production cost of the final glass        is drastically negatively impacted.

Accordingly, there is a demand of the market in the display domain inparticular for a chemically-temperable soda-lime-silica-type glasscomposition, which is more suited for mass production thanaluminosilicate glass, and therefore is available at low cost, and witha base glass/matrix composition that is close to or very similar tocompositions already used in existing mass production.

Next to that, float glass is widely used in general (and increasinglyused in display industry) because of excellence in many respects such asflatness, smoothness and optical quality of surfaces and uniformity ofthickness, and also its relatively low cost of production in comparisonto glass sheets manufactured by other processes like, for example drawnglass.

Unfortunately, when using float glass sheets with small thicknesses, asrequired by the display/electronic market, and applying a chemicalstrengthening by a classical ion-exchange process, a problem called“warping” of the strengthened final glass sheet occurs. This warpingcauses the glass sheet to deform or deviate from flatness and, inparticular, warp is evaluated on the glass sheet after chemicalstrengthening as the ratio d/L, where d equals the distance or depth ofvariation, and L equals the distance or length over which the variationoccurs, as illustrated in FIG. 1. For example, in the case of a floatglass sheet of about 0.7 mm in thickness and a L length of 4 cm, thewarping of the strengthened glass sheet reaches ˜0.04% so that flatnessis seriously compromised. However, in particular in the display domain,such a flatness is highly desirable for many reasons like, for example,to improve optical quality of glass sheets, to allow fitting to adisplay device assembly, to avoid/minimize optical distortion, or alsoto allow uniformly and effective depositing of a coating on the glasssheet (i.e. TCO coatings).

The main reason of warping of float glass during chemical strengtheningcomes from asymmetric faces of float glass from a chemical point of viewand their different behaviour regarding ion-exchange. Indeed, floatglass comprises a so-called “tin face” resulting from the diffusion oftin from the float bath into the lower glass face (in contact with themolten tin). The tin face of a float glass sheet is classically enrichedin tin in the bulk of the glass near the extreme surface, according to aspecific profile (i.e. diffusion profile or with a “hump”) generallyextending over a few microns. Commonly, the face opposite to the tinface is called the “air face”. It has been recognized previously thatthe tin in the lower face of float glass exerts a blocking influence onthe diffusion of ions (i.e. potassium ions) into the lower face of thefloat glass. Next to that, another difference between both faces offloat glass is their availability in sodium ions. Sodium is the speciesthat will be exchanged with potassium during chemical tempering. Ifthere is different sodium availability between each face of float glass,the exchange rate will not be the same for each face. Usually, achemically strengthened float glass sheet thus warps such that the uppersurface (air face, opposite to the tin face) becomes convex.

The warpage of a float glass becomes large with an increase in thedegree of behavior of chemical strengthening. Accordingly, in achemically strengthened float glass having surface compressive stress of600 MPa or more and a depth of a compressive stress layer (DOL) of 15 μmor more, which has been developed to respond to the requirement of highscratch resistance, the problem of warpage becomes obvious compared to achemically strengthened float glass of the related art having surfacecompressive stress (CS) of about 500 MPa and a depth of a compressivestress layer (DOL) of about 10 μm.

Although it is also a clear market demand to use float (ultra-)thinglass sheets in the display domain, there are presently only a fewproposed solutions allowing to avoid at least partially warpingphenomenon in thin float glass, and these proposed solutions have hugedrawbacks which limit their advantageous implementation at an industrialscale. One of the known and classical method/pre-treatment to avoidwarping is to physically grind and then polish the lower face of floatglass in order to eliminate the tin layer before implementingion-exchange treatment. For example, such a pre-treatment is shown inJapanese patent application primary publication No. 58-115043 (1983).Anyway, main existing solutions to warping of thin float glass sheetsinvolve:

-   -   a significant additional cost for the produced glass, and/or    -   an off-line treatment, and/or    -   a higher probability of defects and/or glass breakage, thereby        resulting in a lower production yield (worsening glass final        cost), and/or    -   a loss of the excellent surface created on the surface of the        molten tin bath.

In conclusion, there is a demand of the market in the display domain inparticular for a chemically-temperable soda-lime-silica-type glasscomposition:

-   -   which is more suited for mass production than aluminosilicate        glass, and therefore available at low cost,    -   with a base glass/matrix composition that is close to or very        similar to compositions already used in existing mass        production, and    -   with reduced or even no warping effect after toughening, while        avoiding any additional off-line treatment like polishing or        grinding treatment and thereby keeping excellent surface created        on the surface of the molten tin bath.

3. OBJECTIVES OF THE INVENTION

The objective of the invention in particular is to remedy the citeddisadvantages and resolving the technical problem, i.e. to provide aglass composition which is easy chemically temperable or, in otherwords, more favourable to ion exchange than conventionalsoda-lime-silica glass compositions, and which shows after tougheningreduced warping (to keep flatness), or alternatively controlledincreased warping (to a desired shape).

Another objective of the invention in at least one of its embodiments isto provide a glass composition which is easy chemically temperable andwhich allows reaching strengthening parameters appropriate for a“piece-by-piece” process used to produce cover glass for display devices(edge strength obtained typically by DoL>10-15 microns). In particular,an objective of the invention in such a context is to provide a glasscomposition which is easy chemically temperable and which allowsobtaining great exchange depth, while keeping compressive stress valuesthat result in a better reinforcement of glass.

Another objective of the invention in at least one of its embodiments isto provide a glass composition which is easy chemically temperable andeasy to produce, in particular on an existing line of production ofclassical soda-lime-silica glass. In particular, an objective of theinvention in such a context is to provide a glass composition which iseasy chemically temperable and which does not require long transitiontime when passing from the production of the classical soda-lime-silicacomposition to the temperable composition (and vice-versa). Still insuch a context, an objective of the invention is to provide a glasscomposition which is easy chemically temperable and which does notrequire to use raw materials, techniques and/or industrial installationswhich are different from those employed for classical soda-lime-silicaglass ordinary produced (or, in other words, compatible with classicalfloat process). More particularly, an objective of the invention in atleast one of its embodiments is to provide a glass composition which iseasy chemically temperable and with targeted properties (lowerviscosity, lower working point temperature, melting point<1550-1500° C.,sulfates fining ability, low refractories corrosion, appropriatedevitrification temperature), thereby avoiding known drawbacks ofalumino-silicate composition and making composition compatible withexisting tools for production of soda-lime glass.

Finally, another objective of the invention is to provide a solution tothe disadvantages to the prior art that is simple, quick and, above all,economical.

4. OUTLINE OF THE INVENTION

The invention relates to a float glass sheet having a boron- andlithium-free glass composition comprising the following in weightpercentage, expressed with respect to the total weight of glass:

-   -   65≤SiO₂≤78%    -   5≤Na₂O≤20%    -   0≤K₂O<5%    -   1≤Al₂O₃<6%    -   0≤CaO<4.5%    -   4≤MgO≤12%;    -   (MgO/(MgO+CaO)) ratio≥0.5,    -   the glass sheet having:

$0.01 < {{\frac{\left( {{Na}_{2}O} \right)_{air}}{\left( {{Na}_{2}O} \right)_{tin}} - 1.03}} \leq 3$

Hence, the invention rests on a novel and inventive approach, since itenables a solution to be found for the disadvantages of prior art.

The inventors have indeed found that it is possible to obtain an easychemically temperable glass sheet which is unexpensive and easy to massproduce by combining in a soda-silica glass matrix, a low alumina andCaO content (compared to classical alumina-silicate glass) as well as a(MgO/(MgO+CaO)) ratio which is higher than 0.5 in comparison withclassical soda-lime-silica glass compositions (with typical values forthat ratio below 0.5). Moreover, while combining such a base glassmatrix with a specific ratio between Na₂O amount in the air face and inthe tin face, one can reach an easy chemically temperable glass sheetcapable of having controlled warping through chemical strengthening(reduced warping to keep flatness or, alternatively, controlledincreased warping to get a desired shape).

Throughout the present text, when a range is indicated, the extremitiesare included. In addition, all the integral and subdomain values in thenumerical range are expressly included as if explicitly written. Alsothroughout the present text, the values of content as percentages arevalues by weight (also mentioned as wt %), expressed with respect to thetotal weight of the glass. Moreover, when a glass composition is given,this relates to the bulk composition of the glass, except if explicitlydescribed in another way.

Other features and advantages of the invention will be made clearer fromreading the following description of preferred embodiments given by wayof simple illustrative and non-restrictive examples.

The glass sheet of the invention is made of a soda-silica glasscomposition/matrix, comprising SiO₂ and Na₂O as the main components andfurther comprising MgO, Al₂O₃, etc and optionally CaO, K₂O etc.

The glass sheet of the invention is able to be chemically tempered or,in other words, ion-exchangeable/able to undergo an ion-exchange, withreduced or even no warping effect or alternatively, with increasedwarping to design a shape.

The glass sheet of the invention is a float glass sheet. The term “floatglass sheet” is understood to mean a glass sheet formed by the floatprocess, which consists in pouring the molten glass onto a bath ofmolten tin, under reducing conditions. A float glass sheet comprises, ina known way, a “tin face”, that is to say a face enriched in tin in thebody of the glass close to the surface of the sheet. The term“enrichment in tin” is understood to mean an increase in theconcentration of tin with respect to the composition of the glass at thecore, which may or may not be substantially zero (devoid of tin).Therefore, a float glass sheet can be easily distinguished from sheetsobtained by other glassmaking processes, in particular by the tin oxidecontent which may be measured, for example, by electronic microprobe toa depth of ˜10 microns. In many cases and as illustration, this contentlies between 1 and 5 wt %, integrated over the first 10 microns startingfrom the surface.

The float glass sheet according to the invention may have varied andrelatively large sizes. It can, for example, have sizes ranging up to3.21 m×6 m or 3.21 m×5.50 m or 3.21 m×5.10 m or 3.21 m×4.50 m (“PLF”glass sheet) or also, for example, 3.21 m×2.55 m or 3.21 m×2.25 m (“DLF”glass sheet).

The thickness of the float glass sheet is not particularly limited. Inorder to effectively perform chemical strengthening treatment describedbelow, the thickness of the glass sheet is usually preferably 5 mm orless, more preferably 3 mm or less, more preferably 1.5 mm or less, andparticularly preferably 0.8 mm or less (for example, less than 0.7 mm orless than 0.55 mm or even less than 0.35 mm). The problem of warpageafter chemical strengthening is likely to occur when the thickness ofthe glass sheet is less than 3 mm, and typically, less than 1.5 mm.

According to the present invention, the float glass sheet has:

$0.01 < {{\frac{\left( {{Na}_{2}O} \right)_{air}}{\left( {{Na}_{2}O} \right)_{tin}} - 1.03}} \leq 3$

The value of 1.03 subtracted from the ratio

$\frac{\left( {{Na}_{2}O} \right)_{air}}{\left( {{Na}_{2}O} \right)_{tin}}$

allows eliminating contribution from reference (glass sheet not treatedfor warpage control). The defined term in absolute value allows coveringboth decrease of warpage or controlled increase of warpage.

To obtain the specific Na₂O ratio between air and tin faces in the glasssheet of the invention, a dealkalization treatment is implemented, andthe difference between the degree of dealkalization between that in oneface thereof and that in the other face thereof is set to be within aspecific range. As a result, it is possible to control the exchange rateof ions in a face versus the opposite one, and it is possible to achievea balance in the degree of behaviour of chemical strengthening betweenone face and the other one. For this reason, in the glass sheet of theinvention, it is possible to control the warpage (reduce/avoid thewarpage or alternatively increase the warpage) of the strengthened glasssheet, without conducting grinding or polishing treatment beforestrengthening.

The amount of Na₂O in the air face, namely “(Na₂O)_(air)”, means theNa₂O amount in the bulk of the glass near the extreme surface of the airface. The amount of Na₂O in the tin face, namely “(Na₂O)_(tin)”, meansthe Na₂O amount in the bulk of the glass near the extreme surface of thetin face. According to the invention, the Na₂O amount on each face (tinor air) is measured by an X-ray fluorescence (XRF) spectrometer usingNa-Kα rays. In present text, the amount of Na₂O was determined by usinga calibration curve method build with International glass referencesamples. As the measurement apparatus, S4 Explorer manufactured byBruker is exemplified with following measurement parameters:

Output: Rh 30 kV-100 mA

Filter: No Mask: 34 mm Colimator: 0.46

Analyzing crystal: XS55

Detector: FC Element Rays: Na-Kα

Peak angle (2θ/deg.): 25,017Peak measurement time period (seconds): 30B. G. 1 (2θ/deg.): NAB. G. 1 measurement time period (seconds): 0B. G. 2 (20/deg.): NAB. G. 2 measurement time period (seconds): 0

PHA FC: 37-174.

If the float glass sheet according to the invention is covered by acoating or a layer, the amount of Na₂O is determined while excluding thecoating/layer itself, taking into account the glass only.

Preferably, the float glass sheet has:

$0.03 < {{\frac{\left( {{Na}_{2}O} \right)_{air}}{\left( {{Na}_{2}O} \right)_{tin}} - 1.03}} \leq 1.5$

More preferably, the float glass sheet has:

$0.05 < {{\frac{\left( {{Na}_{2}O} \right)_{air}}{\left( {{Na}_{2}O} \right)_{tin}} - 1.03}} \leq {1\text{,}2}$

Even more preferably, the float glass sheet has:

$0.1 < {{\frac{\left( {{Na}_{2}O} \right)_{air}}{\left( {{Na}_{2}O} \right)_{tin}} - 1.03}} \leq 0.9$

In a very preferred manner, the float glass sheet has:

$0.2 < {{\frac{\left( {{Na}_{2}O} \right)_{air}}{\left( {{Na}_{2}O} \right)_{tin}} - 1.03}} \leq 0.45$

According to a preferred embodiment of the invention, the float glasssheet has:

$\frac{\left( {{Na}_{2}O} \right)_{air}}{\left( {{Na}_{2}O} \right)_{tin}} \leq {1.01.}$

According to this embodiment, preferably, the float glass sheet has:

$\frac{\left( {{Na}_{2}O} \right)_{air}}{\left( {{Na}_{2}O} \right)_{tin}} \leq 1.$

more preferably, the float glass sheet has:

$\frac{\left( {{Na}_{2}O} \right)_{air}}{\left( {{Na}_{2}O} \right)_{tin}} \leq 0.99$

or even

$\frac{\left( {{Na}_{2}O} \right)_{air}}{\left( {{Na}_{2}O} \right)_{tin}} \leq {0.97.}$

The most preferably, the float glass sheet has:

$\frac{\left( {{Na}_{2}O} \right)_{air}}{\left( {{Na}_{2}O} \right)_{tin}} \leq 0.95$

or even,

$\frac{\left( {{Na}_{2}O} \right)_{air}}{\left( {{Na}_{2}O} \right)_{tin}} \leq {0.93.}$

Such embodiments are advantageous as they allow decreasing more and morewarpage through chemical strengthening and thereby keeping as much aspossible flatness of the glass sheet. Some can also lead to negativewarpage (or antiwarpage), which is desirable in some applications.

According to the invention, the composition of the float glass sheet isboron-free. This means that boron is not intentionally added in theglass batch/raw materials and that, if it is present, B₂O₃ content inthe composition of the glass sheet reaches only level of an impurityunavoidably included in the production. For example, B₂O₃ content in thecomposition of the float glass sheet of the invention is less than 0.01or even better less than 0.005 wt %.

According to the invention, the composition of the float glass sheet islithium-free. This means that lithium is not intentionally added in theglass batch/raw materials and that, if it is present, Li₂O content inthe composition of the glass sheet reaches only level of an impurityunavoidably included in the production. For example, Li₂O content in thecomposition of the float glass sheet of the invention is less than 0.01wt % or even better less than 0.005 wt %.

According to the invention, the composition of the float glass sheetcomprises: 1≤Al₂O₃<6 wt %. Preferably, the composition of the floatglass sheet comprises: 1≤Al₂O₃<5 wt % or even: 1≤Al₂O₃<4 wt %. Morepreferably, the composition of the float glass sheet comprises: 1≤Al₂O₃3 wt %. Alternatively, the composition of the float glass sheetcomprises: 2<Al₂O₃<6 wt %. Preferably, the composition of the floatglass sheet comprises: 2<Al₂O₃<5 wt % or even: 2<Al₂O₃<4 wt %. Morepreferably, the composition of the float glass sheet comprises: 2<Al₂O₃3 wt %. Advantageously and alternatively also, 3≤Al₂O₃<6 wt %.Preferably, the composition of the float glass sheet comprises:3≤Al₂O₃<5 wt % or even: 3≤Al₂O₃<4 wt %.

More preferably, the composition of the float glass sheet comprises:4≤Al₂O₃<6 wt % or even 4≤Al₂O₃<5 wt %.

According to the invention, the composition of the float glass sheetcomprises: 0≤CaO<4.5 wt %. Preferably, the composition of the floatglass sheet comprises: 0≤CaO<4 wt % and more preferably, 0≤CaO<3.5 wt %.In a very particularly preferred embodiment, the composition of thefloat glass sheet comprises: 0≤CaO<3 wt %, or even better 0≤CaO<2 wt %.In the most preferred embodiment, the composition of the float glasssheet comprises: 0<CaO<1 wt %.

According to the invention, the composition of the float glass sheetcomprises: 4≤MgO≤12 wt %. Preferably, the composition of the float glasssheet comprises: 5.5≤MgO≤10 wt % and more preferably, 6≤MgO≤10 wt %.

According to the invention, the composition of the float glass sheetcomprises: 0≤K₂O<5 wt %. Preferably, the composition of the float glasssheet comprises: 0≤K₂O<4 wt % and more preferably, 0≤K₂O<3 wt %, evenbetter 0≤K₂O<2 wt %. In the most preferred embodiment, the compositionof the float glass sheet comprises: 1≤K₂O<2 wt %

According to an embodiment, the composition of the float glass sheetcomprises the following: 0.5≤[MgO/(MgO+CaO)]<1. Preferably, thecomposition of the float glass sheet comprises the following:0.6≤[MgO/(MgO+CaO)]<1. More preferably, the composition of the floatglass sheet comprises the following: 0.75≤[MgO/(MgO+CaO)]<1.Alternatively, the composition of the float glass sheet comprises thefollowing: 0.5≤[MgO/(MgO+CaO)]<0.95, or even more better0.5≤[MgO/(MgO+CaO)]<0.85. More preferably, the composition of the floatglass sheet comprises the following: 0.75≤[MgO/(MgO+CaO)]<0.85.

According to a very preferred embodiment, the composition of the floatglass sheet comprises the following: 0.88≤[MgO/(MgO+CaO)]<1. Preferably,the composition of the float glass sheet comprises the following:0.9≤[MgO/(MgO+CaO)]<1. More preferably, the composition of the floatglass sheet comprises the following: 0.9<[MgO/(MgO+CaO)]≤0.95.Alternatively, the composition of the float glass sheet comprises thefollowing: 0.88≤[MgO/(MgO+CaO)]≤0.98. More preferably, the compositionof the float glass sheet comprises the following:0.90≤[MgO/(MgO+CaO)]≤0.98 or even better, 0.92≤[MgO/(MgO+CaO)]≤0.98, oreven more better 0.92≤[MgO/(MgO+CaO)]≤0.95.

According to an embodiment of the invention, the composition comprisestotal iron (expressed in the form of Fe₂O₃) in a content ranging from0.002 to 1.7% by weight. Preferably, the composition of the inventioncomprises a total iron (expressed in terms of Fe₂O₃) content rangingfrom 0.002 to 0.6 wt % and, more preferably, ranging from 0.002 to 0.2wt %.

In a very preferred embodiment, the composition of the inventioncomprises a total iron (expressed in terms of Fe₂O₃) content rangingfrom 0.002 to 0.06 wt %. A total iron (expressed in the form of Fe₂O₃)content of less than or equal to 0.06 wt % makes it possible to obtain aglass sheet with almost no visible coloration and allowing a high degreeof flexibility in aesthetic designs (for example, getting no colorvariation when white silk printing of some glass elements ofsmartphones). The minimum value makes it possible not to be excessivelydamaging to the cost of the glass as such, low iron values often requireexpensive, very pure, raw materials and also purification of these.Preferably, the composition comprises a total iron (expressed in theform of Fe₂O₃) content ranging from 0.002 to 0.04 wt %. More preferably,the composition comprises a total iron (expressed in the form of Fe₂O₃)content ranging from 0.002 to 0.02 wt %. In the most preferredembodiment, the composition comprises a total iron (expressed in theform of Fe₂O₃) content ranging from 0.002 to 0.015 wt %.

According to a particularly preferred embodiment, the composition of thefloat glass sheet of the invention comprises the following in weightpercentage, expressed with respect to the total weight of glass:

-   -   65≤SiO₂≤78%    -   10≤Na₂O≤20%    -   0≤K₂O<4%    -   2<Al₂O₃≤3%    -   0<CaO<3.5%    -   4≤MgO≤12%    -   0.5≤[MgO/(MgO+CaO)]<1

According to this last embodiment, the composition of the float glasssheet of the invention more preferably comprises:

-   -   65≤SiO₂≤78%    -   10≤Na₂O≤20%    -   0≤K₂O<3%    -   2<Al₂O₃≤3%    -   0<CaO<3.5%    -   6≤MgO≤10%    -   0.75≤[MgO/(MgO+CaO)]<1

According to another preferred embodiment, the composition of the floatglass sheet of the invention more preferably comprises:

-   -   65≤SiO₂≤78%    -   10≤Na₂O≤20%    -   0≤K₂O<3%    -   4≤Al₂O₃<5%    -   0<CaO<3.5%    -   6≤MgO≤10%    -   0.88≤[MgO/(MgO+CaO)]<1

According to another embodiment, the composition of the float glasssheet comprises ZnO in a content lower than 0.1 wt % Preferably, thecomposition of the glass sheet is free of ZnO. This means that theelement zinc is not intentionally added in the glass batch/raw materialsand that, if it is present, ZnO content in the composition of the glasssheet reaches only level of an impurity unavoidably included in theproduction.

According to another embodiment, the composition of the float glasssheet comprises ZrO₂ in a content lower than 0.1 wt %. Preferably, thecomposition of the glass sheet is free of ZrO₂. This means that theelement zirconium is not intentionally added in the glass batch/rawmaterials and that, if it is present, ZrO₂ content in the composition ofthe glass sheet reaches only level of an impurity unavoidably includedin the production.

According to still another embodiment, the composition of the floatglass sheet comprises BaO in a content lower than 0.1 wt %. Preferably,the composition of the glass sheet is free of BaO. This means that theelement barium is not intentionally added in the glass batch/rawmaterials and that, if it is present, BaO content in the composition ofthe glass sheet reaches only level of an impurity unavoidably includedin the production.

According to still another embodiment, the composition of the floatglass sheet comprises SrO in a content lower than 0.1 wt %. Preferably,the composition of the glass sheet is free of SrO. This means that theelement strontium is not intentionally added in the glass batch/rawmaterials and that, if it is present, SrO content in the composition ofthe glass sheet reaches only level of an impurity unavoidably includedin the production.

According to still another embodiment, the composition of the floatglass sheet comprises bulk SnO₂ in a content lower than 0.1 wt % (bulkcontent excluding SnO₂ in the “tin face” of a float glass sheet).Preferably, the composition of the glass sheet is free of bulk SnO₂.This means that the element tin is not intentionally added in the glassbatch/raw materials and that, if it is present, bulk SnO₂ content in thecomposition of the glass sheet reaches only level of an impurityunavoidably included in the production.

According to an embodiment of the invention, the composition comprisescoloring components other than iron, chromium and cobalt oxides in atotal content which is less than 0.005 wt %. Such an embodiment allowsto control color and thus to provide a glass sheet which is neutral asmainly requested for display applications. More preferably, thecomposition of the invention comprises coloring components other thaniron, chromium and cobalt oxides in a total content which is less than0.003 wt %.

Advantageously, the composition of the invention may further comprisechromium and/or cobalt oxides in a total content which is between 0.001and 0.025 wt %. This means that the composition may comprise onlychromium, only cobalt or both. Such a specific composition makes theglass especially suitable for touch technology based on IR transmission.

According to one embodiment of the invention, the float glass sheet iscoated with at least one transparent and electrically conducting thinlayer. A transparent and conducting thin layer according to theinvention can, for example, be a layer based on SnO₂:F, SnO₂:Sb or ITO(indium tin oxide), ZnO:Al or also ZnO:Ga.

According to another advantageous embodiment of the invention, the floatglass sheet is coated with at least one antireflection layer. Thisembodiment is obviously advantageous in the case of use of the glasssheet of the invention as front face of a screen. An antireflectionlayer according to the invention can, for example, be a layer based onporous silica having a low refractive index or it can be composed ofseveral layers (stack), in particular a stack of layers of dielectricmaterial alternating layers having low and high refractive indexes andterminating in a layer having a low refractive index.

According to another embodiment, the float glass sheet is coated with atleast one anti-fingerprint layer or has been treated so as to reduce orprevent fingerprints from registering. This embodiment is alsoadvantageous in the case of use of the glass sheet of the invention asfront face of a touchscreen. Such a layer or such a treatment can becombined with a transparent and electrically conducting thin layerdeposited on the opposite face. Such a layer can be combined with anantireflection layer deposited on the same face, the anti-fingerprintlayer being on the outside of the stack and thus covering theantireflection layer.

According to still another embodiment, the float glass sheet is coatedwith at least one layer or has been treated so as to reduce or preventglaring and/or sparkling. This embodiment is of course advantageous inthe case of use of the glass sheet of the invention as front face of adisplay device. Such an anti-glare or anti-sparkling treatment is forexample an acid-etching producing a specific roughness of the treatedface of the glass sheet.

According to the applications and/or properties desired, otherlayer(s)/treatment(s) can be deposited/done on one and/or the other faceof the float glass sheet according to the invention.

The glass sheet of the invention is obtained by a float method. In thefloat method, a glass sheet is manufactured using a melting furnace inwhich a raw material of glass is melted, a float bath in which moltenglass is floated on a molten metal (tin) to form a glass ribbon, and anannealing furnace in which the glass ribbon is annealed. Hereinafter, inthe method description, the term “glass sheet” may be used as a genericterm indicating the glass sheet and/or the glass ribbon.

In an exemplified method of preparing the float glass sheet of theinvention, at least the air face of the glass sheet (or glass ribbon) issubjected to a dealkalization treatment, thereby removing alkalinecomponents, and thus, reaching the specific ratio according to theinvention. For example, the dealkalization method may advantageously bea method of treating the glass with a substance capable of ion exchangereaction(s) with alkaline components in the glass. As a substancecapable of ion exchange reaction(s) with alkaline components in theglass, examples include molecules having fluorine atoms in the structurethereof, sulphur-based compounds, acid, or nitride. The substancecapable of ion exchange reaction(s) with alkaline components in theglass may be for example in the form of gas, liquid, . . . or any othersuitable form (available form(s) depend(s) amongst others of thesubstance itself).

Examples of substance containing molecules having fluorine atoms in thestructure thereof include hydrogen fluoride (HF), freon (for example,chlorofluorocarbon, fluorocarbon, hydrochlorofluorocarbon,hydrofluorocarbon, halon and the like), hydrofluoric acid, fluorine(simple substance), trifluoroacetic acid, carbon tetrafluoride, silicontetrafluoride, phosphorus pentafluoride, phosphorus trifluoride, borontrifluoride, nitrogen trifluoride, chlorine trifluoride, and the like.

Examples of sulphur-based compounds include sulfurous acid, sulfuricacid, peroxomonosulfuric acid, thiosulfuric acid, dithionous acid,disulfuric acid, peroxodisulfuric acid, polythionic acid, hydrogensulfide, sulfur dioxide, and the like.

Examples of an acid include hydrochloric acid, carbonic acid, boricacid, lactic acid, and the like.

Examples of a nitride include nitric acid, nitric monoxide, nitrogendioxide, nitrous oxide, and the like.

The method for application of the substance capable of ion exchangereaction(s) with alkaline components in the glass may be chosendepending on the form of the substance and any other suitable anddesired parameter.

In the float process in which glass is formed on a molten metal (tin)bath, the substance capable of ion exchange reaction(s) with alkalinecomponents in the glass may be supplied to the glass sheet beingconveyed on the molten metal bath from the side not in contact with themetal surface, thereby treating the top face of the glass sheet/ribbon(air face). In the annealing zone subsequent to the molten metal (tin)bath, the glass sheet is conveyed by roller conveying. Here, theannealing zone includes not only the inside of the annealing furnace butalso a portion where the glass sheet is conveyed from the molten metalbath to the annealing furnace in the float bath. In the annealing zone,the substance capable of ion exchange reaction(s) with alkalinecomponents in the glass may be supplied from the face that was not incontact with the molten metal (air face) and/or the opposite face (tinface).

The invention also relates to the use of the chemically tempered floatglass sheet according to the invention in an electronic device.

EXAMPLES

Powder raw materials were mixed together and placed in meltingcrucibles, according the compositions specified in Table 1. The rawmaterial mix was then heated up in an electrical furnace to atemperature allowing complete melting of the raw material.

After the melting and the homogenization of the composition, the glasswas cast in several small samples of 40*40 mm and annealed in anannealing furnace. Subsequently, the samples were polished up to asurface state similar to floated glass (mirror polishing). Severalsamples were produced for each composition.

Composition of comparative example 1 corresponds to a classical low-ironsoda-lime (SL) glass according to the state of the art and compositionof comparative example 2 corresponds to a commercially availablealumino-silicate (AS) glass.

Compositions of examples 1-10 correspond to compositions according tothe invention.

Two glass samples from each composition from examples were then treatedwith a dealkalization substance: samples were pre-heated at 200° C.inside an electric furnace. They were then heated-up in anotherelectrical furnace up to 450° C. The heating-up step took 40 min. Athermocouple placed inside the furnace allowed checking the temperatureof the sample before and after the dealkalinization treatment.

TABLE 1 Comp. Comp. ex. 1 ex. 2 Wt % (SL) (AS) Ex. 1 Ex. 2 Ex. 3 Ex. 4Ex. 5 Ex. 6 Ex. 7 Ex. 8 Ex. 9 Ex. 10 SiO₂ 72 60.9 72.1 71.3 70.6 70.268.4 69.4 70.5 74.0 73.9 73.1 Al₂O₃ 1.3 12.8 2.0 2.3 3.0 2.9 4.9 3.8 3.11.1 1.2 1.1 MgO 4.5 6.7 7.5 6.7 9.1 9.2 8.8 8.3 7.2 8.9 8.9 6.9 CaO 7.90.1 3.0 4.3 0.6 0.6 1.0 1.1 3.4 0.3 0.4 3.6 Na₂O 13.9 12.2 15.0 13.616.3 15.2 16.5 16.0 13.8 15.3 15.3 14.9 K₂O 0 5.9 0 1.4 0 1.2 0.02 0.951.7 0.2 0.2 0.2 Fe₂O₃ 0.01 0 0.01 0.01 0.01 0.01 0.01 0.01 0.09 0.091.10 1.10 BaO 0 0.2 0 0 0 0 0 0 0 0 0 0 SO₃ 0.36 0 0.36 0.36 0.36 0.360.36 0.36 0.36 0.36 0.36 0.36 SrO 0 0.2 0 0 0 0 0 0 0 0 0 0 ZrO₂ 0 1.0 00 0 0 0 0 0 0 0 0

In order to insure the same thermal history for all samples, the sampleswere maintained at 450° C. during 20 minutes. The dealkalizationtreatment consisted in injecting SO₂ (10% SO₂+90% dried air) at 20 l/hwith humidified air at 2 l/h (bubbling of dried air inside demi-water at25° C.) selectively on a glass face 1 (then called the “treated face 1”)for a given time (t=0 (reference), 3, 10, 17 or 20 minutes), leading todifferent levels of soda depletion. The dealkalization step is done sothat this step happens during but at the end of the maintain at 450° C.(see for example, FIG. 2 for temperature profile for 10 minutes ofdealkalization treatment), in order to avoid any relaxation effect andretro diffusion of soda from the bulk towards the surface.

The samples were then removed from the heating furnace, washed andanalyzed: one sample was used for XRF measurements of composition of theglass bulk and Na₂O amount on the treated glass face 1; the other onehas undergone chemical tempering.

Chemical Tempering

Chemical tempering #1: Some samples prepared in above section werechemically tempered at the same time and in the same conditions. Thesamples of different compositions were placed in a cassette, preheatedand then dipped in a molten KNO₃ (>99%) bath at 420° C. for 220 minutes.

Chemical tempering #2: Some samples prepared in above section werechemically tempered at the same time and in the same conditions. Thesamples of different compositions were placed in a cassette, preheatedand then dipped in a molten KNO₃ (>99%) bath at 430° C. for 240 minutes.

After the ion exchange, the samples were cooled down and washed.Subsequently the surface compressive stress (CS) and the depth ofexchanged layer (DoL) were measured via photoelasticimetry.

Table 2 summarizes, for chemical tempering #1, the average value of CSand DoL from treated face 1, for random samples of each of examples 1-4and 7-10 according to the invention and Comparative examples 1-2.

TABLE 2 Comp. Comp. chemical ex. 1 ex. 2 tempering #1 (SL) (AS) Ex. 1Ex. 2 Ex. 3 Ex. 4 Ex. 7 Ex. 8 Ex. 9 Ex. 10 Surface 861 884 785 803 820818 808 645 646 693 compressive stress (MPa) Depth of 6.7 36.1 13.3 11.516.6 18.1 13.9 20.3 18.6 12.6 exchanged layer (μm)

Those results show that combining, in a soda-silica glass matrix, a lowalumina and CaO content as well as a (MgO/(MgO+CaO)) ratio which ishigher than 0.5 allows to significantly improve the depth of exchangedlayer, while keeping a high surface compressive stress and thus, toincrease the glass reinforcement.

Moreover, DOL values of compositions according to the invention are wellappropriate for a “piece-by-piece” process used to produce cover glassfor display devices (preferably higher than 10 microns and verypreferably higher than 12 microns or even better higher than 15microns).

Table 3 summarizes, for chemical tempering #2 and for the treated face1, Na₂O amount as well as the average value of CS and DoL for randomsamples of examples 5-6 according to the invention and comparativeexample 1, depending on the duration of dealkalization treatment. Table3 also shows the same values/parameters obtained for the other face 2(non-treated by a dealkalization treatment), as such (case 1) oralternatively, while simulating a “tin face” coming from a float process(case 2).

TABLE 3 chemical tempering #2 Comp. ex. 1 (SL) Ex.5 Ex.6 Dealk. duration(min) 0 3 10 20 0 3 10 20 0 3 10 20 Treated face 1 Na₂O on treated 13.5412.97 12.74 12.51 16.24 15.66 14.70 14.46 15.49 15.38 13.51 14.07 face(%) Surface compressive 662 617 578 592 785 771 730 715 695 714 637 678stress (MPa) Depth of exchanged 11.9 11.8 11.6 11.7 25.4 25.3 25.6 25.830.9 29.5 30.0 29.4 layer (μm) Case 1: Non-treated face 2 Surfacecompressive 662 662 662 662 785 785 785 785 695 695 695 695 stress (MPa)Depth of exchanged 11.9 11.9 11.9 11.9 25.4 25.4 25.4 25.4 30.9 30.930.9 30.9 layer (μm) Computed warpage 0.000 −0.019 −0.038 −0.031 0.000−0.014 −0.041 −0.049 0.000 −0.010 −0.074 −0.046 (%) Case 2: Non-treatedface 2 with float simulation (tin face) Na₂O on non-treated 13.1 13.113.1 13.1 15.8 15.8 15.8 15.8 15.0 15.0 15.0 15.0 face (%)${\frac{\left( {{Na}_{2}O} \right)_{air}}{\left( {{Na}_{2}O} \right)_{tin}} - 1.03}$0 −0.04 −0.06 −0.08 0.00 −0.04 −0.10 −0.11 0 −0.01 −0.13 −0.09 Surfacecompressive 722 722 722 722 856 856 856 856 758 758 758 758 stress (MPa)Depth of exchanged 10 10 10 10 21 21 21 21 26 26 26 26 layer (μm)Computed warpage 0.022 0.003 −0.016 −0.009 0.050 0.036 0.009 0.001 0.0510.041 −0.023 0.005 (%)

Firstly, one can observe that the dealkalization treatment (treatedface 1) allows to tune the chemical tempering performance of the treatedface, enabling a controlled warpage. The dealkalinization treatmentdecreases a little bit CS but while keeping high performances on theDOL. Nevertheless, the CS levels obtained even with the dealkalizationtreatment with composition of the invention remains globally higher thanthe soda-lime reference (comparative example 1).

Next to that, as a result of CS modification due to the dealkalinizationtreatment, the mechanical constraints induced by the chemical temperingon the treated face 1 will evolve comparing to the other face 2. Whileevolving, one could play on this equilibrium of mechanical constraintsin order to obtain the desired warpage or suppress it. This isillustrated in Table 3 by evaluation of the mechanical constraints andinduced warpage on examples 5-6 and comparative example 1. The warpagewas computed mainly based on CS, DOL and sample geometrical, dimensionsfor a square glass sheet (0.7 mm thickness, 4×4 cm), as being theelevation of the middle of the side versus the centre of the samplewhile convex face is down (d/L, see FIG. 1). A negative value of warpagemeans that treated face is concave, while a positive value means thatthe treated face is convex.

Case 1 in table 3 shows the evolution of the warpage when face 1 istreated by a dealkalization and face 2 is as such (untreated). Such acase mimics an industrial case in which the glass sheet is produced bythe float process but wherein the “tin face” has been polished (face 2)and the “air face” has been treated by a dealkalization treatment (face1), prior to tempering. One can observe the variation of the warpagewhile applying different time of dealkalinization treatment, therebymodifying the Na₂O available content, on the treated face 1 versus thenon-treated face 2.

Case 2 in table 3 shows the evolution of the warpage when face 1 istreated by a dealkalization and face 2 is untreated but corresponds to asimulated “tin face” coming from a float process. Such a case mimics anindustrial case in which the glass sheet is produced by the floatprocess (face 2 is “tin face”) and the “air face” has been treated by adealkalization treatment (face 1), prior to tempering. In referenceconditions, the ratio Na₂O air/tin is 1.03. Also, the CS level isclassically 9% higher on tin face than on air face and DOL is 17% loweron tin face than on air face. This allows for each composition kind toestablish a reference tin face. From the results, one can clearlyobserve the presence of a significant warpage for examples 5-6 andcomparative example 1 while no warpage treatment is applied(duration=0). Next, one can also observe that the initial warpage can becontrolled/modified/suppressed by controlling the amount of Na₂O in theair face versus in the tin face, in the range claimed.

Other Properties

The following properties were evaluated on the basis of glasscomposition using Fluegel model (Glass Technol.: Europ. J. Glass Sci.Technol. A 48 (1): 13-30 (2007); and Journal of the American CeramicSociety 90 (8): 2622 (2007)) for compositions of examples 1-4 accordingto the invention as well as of Comparative examples 1-2:

-   -   Glass melt density evaluated at 1200 and 1400° C.;    -   Viscosity through the “Melting point temperature T2”;    -   “Working point temperature T4”;    -   Devitrification temperature T0;    -   Coefficient of thermal expansion (CET);

Moreover, refractories corrosion behaviour was evaluated according tothe known “Dunkl corrosion test” (during 36 h at 1550° C.), given inpercentage corresponding to the loss of material at the metal line.

In a general manner:

The melting point temperature T2 is preferably at most 1550° C., morepreferably at most 1520° C., the most preferably at most 1500° C.

The Working point temperature T4 is preferably at most 1130° C., morepreferably at most 1100° C., the most preferably at most 1070° C.

The devitrification temperature T0 is preferably at most T4, morepreferably at most T4-20° C., the most preferably at most T4-40° C.

The loss of material at the metal line during corrosion test ispreferably less than 13%, more preferably less than 11%, the mostpreferably less than 9%.

CET value (in 10⁻⁶/K) is preferably at most 9.6 and more preferably atmost 9.4.

Table 4 summarizes these properties for examples 1-4 and 7-10 accordingto the invention and Comparative examples 1-2.

The compositions according to present invention are suitable for formingby a float process and while using existing furnace tools for productionof soda lime glass because of:

their melting point temperature T2 being lower than 1500° C. and whichare comparable to a classical soda lime glass (Comparative ex. 1) andsignificantly lower compared to an aluminosilicate glass (Comparativeex. 2);

their working point temperature T4 which is lower than 1100° C. andwhich are comparable to a classical soda lime glass (Comparative ex. 1)and lower compared to an aluminosilicate glass (Comparative ex. 2);

their devitrification temperature T0 are suitable because lower thanworking point temperature T4;

their glass density which is very close to soda lime and aluminosilicateglasses (Comparative ex. 1-2), thereby avoiding/limiting density defectsduring composition change (transition);

their good results in term of refractory corrosion, better than aclassical soda lime glass (Comparative ex. 1).

Moreover, the compositions according to present invention havecoefficients of thermal expansion (CET) which reach in a known mannerappropriate values for a subsequent chemical tempering (limitingdifferentiated cooling deformation phenomenon). More specifically, thecompositions according to present invention show better (lower) valuesfor CET than aluminosilicate glass and thus are less sensitive todifferentiated cooling issues than AS glass.

TABLE 4 Comp Comp ex. 1 ex. 2 (SL) (AS) Ex. 1 Ex. 2 Ex. 3 Ex. 4 Ex. 7Ex. 8 Ex. 9 Ex. 10 Glass at 2.37 2.32 2.34 2.35 2.33 2.33 2.35 2.33 2.332.34 melt 1200° C. density at 2.34 2.32 2.32 2.33 2.32 2.32 2.33 2.312.31 2.32 1400° C. Melting point T2 1463 1601 1484 1492 1485 1493 14991485 1487 1478 (° C.) Working point T4 1037 1176 1050 1055 1048 10531058 1047 1050 1044 (° C.) Devitrification 994 951 958 968 989 1028 986989 990 932 temperature T0 (° C.) CET @210° C. (10⁻⁶/ 9.15 9.68 9.079.17 9.33 9.37 9.23 8.90 8.93 9.10 K) Dunkl test 12.10 — 7.69 5.53 —5.29 — — — — 36 h/1550° C. (%)

Finally, compositions according to the invention allow to get sulfatefining ability during their manufacture/melting, thanks to an adequatesolubility of sulfate and suitable high-temperature viscosity.

1. A float glass sheet having a boron- and lithium-free glasscomposition comprising the following in weight percentage, expressedwith respect to the total weight of glass: 65≤SiO₂≤78% 5≤Na₂O≤20%0≤K₂O<5% 1≤Al₂O₃<6% 0≤CaO<4.5% 4≤MgO≤12% a (MgO/(MgO+CaO)) ratio≥0.5characterized in that the glass sheet has:$0.01 < {{\frac{\left( {{Na}_{2}O} \right)_{air}}{\left( {{Na}_{2}O} \right)_{tin}} - 1.03}} \leq 3.$2. A float glass sheet according to claim 1, wherein the compositioncomprises total iron (expressed in the form of Fe₂O₃) in a contentranging from 0.002 to 1.7% by weight.
 3. A float glass sheet accordingto claim 1, wherein the composition comprises total iron (expressed inthe form of Fe₂O₃) in a content ranging from 0.002 to 0.06% by weight.4. A float glass sheet according to claim 1, wherein the compositioncomprises total iron (expressed in the form of Fe₂O₃) in a contentranging from 0.002 to 0.02% by weight.
 5. A float glass sheet accordingto claim 1, wherein the composition comprises: 1≤Al₂O₃<5 wt %.
 6. Afloat glass sheet according to claim 1, wherein the compositioncomprises: 1≤Al₂O₃<4 wt %.
 7. A float glass sheet according to claim 1,wherein the composition comprises: 1≤Al₂O₃<3 wt %.
 8. A float glasssheet according to claim 1, wherein the composition comprises: 2<Al₂O₃<6wt %.
 9. A float glass sheet according to claim 1, wherein thecomposition comprises: 2<Al₂O₃<4 wt %.
 10. A float glass sheet accordingto claim 1, wherein the composition comprises: 0.5≤[MgO/(MgO+CaO)]<1.11. A float glass sheet according to claim 1, wherein the compositioncomprises: 0.75≤[MgO/(MgO+CaO)]<1.
 12. A float glass sheet according toclaim 1, wherein the composition comprises: 0.88≤[MgO/(MgO+CaO)]<1. 13.A float glass sheet according to claim 1, wherein the glass sheet has:$\frac{\left( {{Na}_{2}O} \right)_{air}}{\left( {{Na}_{2}O} \right)_{tin}} \leq 1.$14. A float glass sheet according to claim 1, wherein the glass sheet ischemically tempered.
 15. An electronic device, comprising the floatglass sheet of claim 1.